All around the world, cities are spreading out into the surrounding land — but nature is unexpectedly asserting itself in the heart of the metropolises, as well. A number of carnivores are not just adapting to cities around the globe, but actually thriving. And meanwhile some urban trees can grow as much as eight times faster than their rural counterparts.

We talked to some experts on urban wilderness — and learned that the cities of the future may be where the wild things are.

Cities are among the most challenging realms for carnivorous mammals to live, lacking much in the way of sheltering vegetation and other natural resources. Still, as cities grow, the habitats for these animals are disappearing worldwide. As their natural homes vanish, many carnivores have rooted themselves in metropolises as a means of survival.

"If urban co-existence with humans is the future for many species of carnivore, it is important that we know as much about it as possible," behavioral ecologist Philip William Bateman at the University of Pretoria in South Africa tells io9. "Animals in urban environments will either adapt or become extinct — with the spread of urbanization, this is the future for most species."

Wild creatures in the city

The red fox, coyote, raccoon, Eurasian badger and other medium-sized carnivores not only survive in cities, but have managed to prosper, living off garbage, fruit, rodents, birds, pets, livestock, roadkill and food that people intentionally leave out for them. For instance, during the 1990s, there was a 15-fold increase in the numbers of coyotes removed annually from the Chicago metropolitan area. And Florida raccoons have reached a population density between four and 400 times greater than their rural cousins.

Meanwhile, even though large carnivores such as bears, wolves and hyenas may not live permanently within cities, they can nevertheless significantly benefit from living close to them. For example, there has been a 10-fold increase in complaints about black bears venturing into urban areas of Nevada.

Why do some carnivores fare so much better than others in cities? Having a general diet is key. Body size is another issue — large sizes can help creatures survive when traveling from one scrap of habitat to another, but getting too large can also put too much of a strain on any resources they find within these fragments.

Still, in the very long term, cities that seem good for carnivores may be very bad for them indeed. "Black bears in Nevada appear to be attracted to towns, [but they] become fatter and breed earlier there. [They] die young and cannot [breed] enough young to replace these deaths. Cities are death traps for that species," Bateman says.

Research on how animals adapt to cities could point to new ways to help us conserve these endangered carnivores. For instance, "British red foxes like urban areas with big, well-vegetated gardens," Bateman tells io9. "The trend to smaller gardens and the splitting up of old houses into apartments with more people and more cars bodes ill for foxes. Maintaining even relatively small 'green lung' areas might mean providing enough resources to keep these animals in urban areas — if, of course, people do want them there."

Conserving urban carnivores can be a tricky business. "In Australia, for example, urban environments may be important for species such as bandicoots, possums and various reptiles, but they are also good for the introduced red fox, which prey on all these," Bateman says. "Being able to identify what is good for natives and bad for invasive [species] would be invaluable."

While such work could help conserve these animals, "urban carnivores are still carnivores — best stay away from them," Bateman cautions. "They are not domesticated. They are choosing resources near humans, they are not choosing to be friendly to humans." Bateman and his colleague Patricia Fleming detailed these findings online April 19 in the Journal of Zoology.

City trees grow eight times faster.

Scientists have also analyzed how plants are faring in cities, focusing on the red oak. These trees and their close relatives dominate areas, ranging from northern Virginia to southern New England. Researchers planted red oak seedlings in northeastern Central Park, in two forest plots in the suburban Hudson Valley, and near the city's Ashokan Reservoir, in the Catskill foothills, some 100 miles north of Manhattan. All the trees were given fertilizer and weekly watering.

Large cities are hotter than surrounding countryside — a well-known phenomenon known as the urban heat island effect, that is a result of solar energy getting absorbed by pavement, buildings and other infrastructure, and then radiated back into the air. The city seedlings experienced maximum daily temperatures averaging more than 4 degrees F higher, and minimum averages were more than 8 degrees F higher.

After growing from May to August, the city seedlings had grown eight times more biomass than the country ones, mainly by putting out more leaves. "We never suspected the response would be so dramatic," researcher Kevin Griffin, a tree physiologist at Columbia University's Lamont-Doherty Earth Observatory, tells io9. "I'm happy to know that young native trees are doing well in the city, and that all the hard work the parks department and the Central Park Conservancy does to plant and maintain so many trees appears to be supported by a favorable growing environment."

The city also has higher levels of airborne nitrogen, a fertilizer, due to air pollution — which could have helped the trees as well. Still, temperature seemed to be the main factor behind their super-charged growth. The scientists detailed their findings online April 5 in the journal Tree Physiology.

"People should be aware that urbanization has impacts on living organisms that are not always intuitive or simple to predict," Griffin says. "For example, I'm sure most people first think of how hot the day was — the maximum temperature — when they consider the effect of temperature or temperature change, yet our work suggests that the main response may be to nighttime temperatures."

Still, "people should not think that accelerated growth in young native trees translates to larger mature trees," Griffin says. "We just don't know if that is the case, but certainly the large trees in Central Park are not eight times larger than the mature trees I work on in Black Rock Forest, one of our rural sites."

Given how half the human population now living in cities, understanding how nature will interact with metropolises is key. "With human influence spreading across the globe, nature and urban environments are inseparable," Griffin says. "Plants can adapt to these changes in their environments, and in this case, really thrive in a human environment."

Also, with temperatures projected to rise globally, "cities are special places — they might be laboratories for what the world will look like in coming years," forest ecologist Gary Lovett at the Cary Institute of Ecosystem Studies in upstate New York said in a statement. "What kinds of trees are doing well there now might be related to what kinds might do well up here, in a number of years."